The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Increasing fuel economy is a desirable goal for auto manufacturers. Consumers desire high fuel economy without sacrificing performance. Turbocharging provides a method for increasing performance during demanding conditions while reducing the overall fuel economy of the vehicle since a smaller displacement engine can be used.
One type of turbocharging system is a two-stage series sequential turbocharger. In such a two-stage system, a high-pressure turbine and a low-pressure turbine are provided in-series. When the engine is working at high load or high speed or both, the turbocharging system may use only the low-pressure turbine while bypassing the high-pressure turbine. When the engine load and speed are not high, the high-pressure turbine and low-pressure turbine work together in-series.
Sequential turbochargers typically include a bypass valve that is controlled differently for different engine conditions. The actuation strategy for the valve is important for smooth transient controls and steady state performance. The bypass valve has a significant flow leakage at its nominal closed position which creates an indeterminate condition for control and operation.